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Memory Access and
Instruction Execution
Timing
AT90S/LS2323/2343
16
This section describes the general access timing concepts for instruction execution and
internal memory access.
The AVR CPU is driven by the System Clock Ø, directly generated from the external
clock signal applied to the CLOCK pin. No internal clock division is used.
Figure 21. shows the parallel instruction fetches and instruction executions enabled by
the Harvard architecture and the fast-access register file concept. This is the basic pipe-
lining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for
functions per cost, functions per clocks and functions per power unit.
Figure 21. The Parallel Instruction Fetches and Instruction Executions
System Clock Ø
1st Instruction Fetch
1st Instruction Execute
2nd Instruction Fetch
2nd Instruction Execute
3rd Instruction Fetch
3rd Instruction Execute
4th Instruction Fetch
Figure 22. shows the internal timing concept for the register file. In a single clock cycle
an ALU operation using two register operands is executed and the result is stored back
to the destination register.
Figure 22. Single Cycle ALU Operation
System Clock Ø
Total Execution Time
Register Operands Fetch
ALU Operation Execute
Result Write Back
The internal data SRAM access is performed in two System Clock cycles as described
in Figure 23..
T1
T2
T1
T2
T3
T4
T3
T4
1004D–09/01
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